Project Summary/Abstract Cerebrovascular disease of the central nervous system (CNS) remains a leading cause of morbidity and mortality in the US. Ischemic injury following vascular occlusion is often dictated by the extent of endothelial cell loss and subsequent repair and remodeling of pre-existing collateral vessels. One approach to minimizing the extent of neural damage is to encourage collaterogenesis as a means of restoring blood flow to the vulnerable adjacent tissue. An extensive vascular network has been shown to support collateral blood flow and mitigate the resulting neurovascular defects; however, the mechanism(s) regulating native formation and injury-induced re-establishment of the collateral vascular network are largely unknown. The current proposal outlines proof-of-principle experiments that will advance our understanding of how ephrin/Eph signaling may Recently, EphA4 receptor loss-of-function studies demonstrate robust surface pial arteriole collateral vessel formation. These and other preliminary data are one of the first demonstrations of increased collateral vessel numbers in the cerebral cortex following gene ablation. Based on previous and preliminary experiments, we propose to test the hypothesis that EphA4 receptor is upregulated in the cortex after ischemic stroke and negatively regulates collaterogenesis, thereby limiting restoration of blood flow and tissue preservation. In Aim 1 we will examine the expression pattern of EphA4 on surface pial arterioles and endothelial associated astrocytes in the cortex of nave, sham and ischemic-injured mice. In Aim 2 we will evaluate how cell-specific EphA4 deletion on endothelial cells and/or astrocytes effects collaterogenesis, post-occlusion blood flow and infarct volume after stroke. Finally, we will determine whether therapeutic delivery of commercially available blockers of EphA4 also can promote collateral blood flow and impede the process of collaterogenesis. repair in Aim 3. Overall, these studies will assess t he functional role of EphA4 on collaterogenesis of surface This investigation will also add some exciting progress in basic and translational vascular research. Thus, the proposed research is relevant to the NIH's mission to foster innovative research strategies to advance the nation's capacity to improve health and expose research trainees to hands on experience in a biomedical laboratory setting. pial arterioles using both transgenic mice and drug delivery approaches.